Tube fighting is a sport developed by our family, and taken to its heights by the big boys. There are no rules. The object is to knock the other guy over.
Most damage can be done in the 'Slide'. That's where the tubes respond to a sharp turn and bounce from one side of the wake to the other. The receiving tube must be well leaned into the turn. The attacking tube must control the speed and hit at the right moment, preferably when the other tube is bouncing high on a wave.
Update: Forgot the conclusion. Big guy wins, other guy meat.